The present invention relates to nested ceramic fibers, non-circular ceramic fibers, and a piggyback micro-mold method for producing both kinds of ceramic fibers as well as circular ceramic fibers.
It is desirable to provide ceramic material in a fiber form so that the ceramic fibers can be suspended in a binder and more easily processed to form a ceramic/binder composite material. However, the production of ceramic fibers has been expensive. Ceramic refractory fibers of alumina-silica, alumina and zirconia can be produced by a blowing method which calls for shattering a stream of molten ceramic with a jet of air or steam. Production of ceramic fibers by this method has the disadvantage of requiring large amounts of capital investment in process and control machinery. Other methods which have been used to produce ceramic fibers include spinning methods, continuous filament methods, colloidal evaporation processes, vapor deposition single-crystal method, whisker method, oxidation method, crystallization method and pseudomorphic alteration method.
Many ceramic materials that would be desirable in the form of fibers are not spinnable. Most ceramic fibers presently made are limited by the process of drawing the fibers from a melt and rapidly cooling to prevent devitrification or crystallization. Phase separation can also be a problem. This is particularly true in the case of glass fibers. Ceramic fibers produced by melt drawing are further limited to compositions containing appreciable amounts of so-called "glass forming oxides" such as Si0.sub.2, B.sub.2 0.sub.5, or P.sub.2 0.sub.5. Melt drawing requires high temperatures and special equipment for the drawing of the fibers.
Synthesis of single or multicomponent oxides via a sol-gel process has been possible since at least 1969 when Dislich and Hinz developed a chemical basis for the preparation of multicmmponent oxides. The formation of glasses and ceramics via the sol-gel process results in very homogeneous, high purity materials from the mixing that occurs on the molecular scale. The glasses can be formed using the sol-gel process using relatively low temperatures. The absence of phase separation and crystallization during sol-gel processing allows it to be used to produce glasses and ceramics from compositions that would exhibit phase separation and crystallization during conventional melting processes.
However, sol-gel processing does pose a number of difficulties. For example, one drawback to sol-gel processing is the difficulty in forming large monolithic pieces. Many compositions formed via the sol-gel process are limited to powders or thin films.
Thus, the formation of fibers by spinning or drawing from a sol is expensive and difficult depending on the material to be spun or drawn.
Composite materials including graphite fibers in a matrix can be formed, and composite materials can be formed with ceramic fibers in a matrix as in U.S. Pat. No. 4,454,190. However, ceramic materials are difficult to work with because of their amorphous properties prior to sintering and their brittle properties after sintering. Moreover, the production of a fiber having a ceramic portion and a non-ceramic portion is not known.